University of Crete HEP Seminars


FP7

Recordings can be found in this link


Hydrodynamic diffusion and its breakdown near AdS2 fixed points

Speaker: Blaise Goutéraux
Institution: École Polytechnique
Time: Friday 08 January 2021, 13:00
Venue: Webinar
Abstract: Hydrodynamics provides a universal description of interacting quantum field theories at sufficiently long times and wavelengths, but breaks down at scales dependent on microscopic details of the theory. We use gauge-gravity duality to investigate the breakdown of diffusive hydrodynamics in two low temperature states dual to black holes with AdS2 horizons. We find that the breakdown is characterized by a collision between the diffusive pole of the retarded Green's function with a pole associated to the AdS2 region of the geometry, such that the local equilibration time is set by infra-red properties of the theory. The absolute values of the frequency and wavevector at the collision (ωeq and keq) provide a natural characterization of all the low temperature diffusivities D of the states via D=ωeq/keq^2 where ωeq=2πΔT is set by the temperature T and the scaling dimension Δ of an infra-red operator. We confirm that these relations are also satisfied in an SYK chain model in the limit of strong interactions.
Recording: The recorded talk can be found here.

Slow scrambling in extremal BTZ and microstate geometries

Speaker: Ben Craps
Institution: VUB, Bruxelles
Time: Tuesday 12 January 2021, 13:00
Venue: Webinar
Abstract: Out-of-time-order correlators (OTOCs) that capture maximally chaotic properties of a black hole are determined by scattering processes near the horizon. This prompts the question to what extent OTOCs display chaotic behaviour in horizonless microstate geometries. This question is complicated by the fact that Lyapunov growth of OTOCs requires nonzero temperature, whereas constructions of microstate geometries have been mostly restricted to extremal black holes. We compute OTOCs for a class of extremal black holes, namely maximally rotating BTZ black holes, and show that on average they display "slow scrambling", characterized by cubic (rather than exponential) growth. Superposed on this average power-law growth is a sawtooth pattern, whose steep parts correspond to brief periods of Lyapunov growth associated to the nonzero temperature of the right-moving degrees of freedom in a dual conformal field theory. Next we study the extent to which these OTOCs are modified in certain "superstrata", horizonless microstate geometries corresponding to these black holes. Rather than an infinite throat ending on a horizon, these geometries have a very deep but finite throat ending in a cap. We find that the superstrata display the same slow scrambling as maximally rotating BTZ black holes, except that for large enough time intervals the growth of the OTOC is cut off by effects related to the cap region, some of which we evaluate explicitly.
Recording: The recorded talk can be found here.

Entanglement entropy in expanding spacetimes

Speaker: Wilke van der Schee
Institution: CERN
Time: Tuesday 19 January 2021, 13:00
Venue: Webinar
Abstract: This talk will be about entanglement entropy in empty 4-dimensional de Sitter spacetime of a non-conformal QFT [2011.08194]. I will first briefly describe the set-up and show how a hydrodynamic plasma dilutes and falls out of equilibrium due to expansion towards empty de Sitter spacetime. We track the evolutions of the event and apparent horizons, showing in particular that the temperature of the event horizon approaches the de Sitter temperature. Interestingly, in the empty setting we can show that extremal surfaces in the holographic dual of spherical entangling regions on the boundary QFT probe beyond the dual event horizon if and only if the entangling region is larger than the cosmological horizon.
Recording: The recorded talk can be found here.

Bootstrapping Quantum Field Theories

Speaker: João Penedones
Institution: EPFL, Lausanne
Time: Tuesday 26 January 2021, 13:00
Venue: Webinar
Abstract: I will review the recent revival of the S-matrix Bootstrap program. In particular, I will discuss the formulation that includes scattering amplitudes, form factors and spectral densities.
Recording: The recorded talk can be found here.

The swampland at large number of space-time dimensions

Speaker: Dieter Lüst
Institution: Max-Planck-Institute, Munich
Time: Tuesday 02 February 2021, 13:15
Venue: Webinar
Abstract: We discuss some aspects of swampland constraints - especially the swampland distance conjecture - in a large number of space-time dimensions D. We analyze Kaluza-Klein (KK) states at large D and find that some KK spectra possess an interesting dependence on D. On the basis of these observations we propose a new large dimension conjecture. We apply it to KK states of compactifications to anti-de Sitter backgrounds where it predicts an upper bound on the dimension of space-time as a function of its characteristic radius. We also apply our conjecture to black hole spacetimes, whose entropies have a D-dependence very similar to that of the KK spectrum.
Recording: The recorded talk can be found here.

S-matrix constraints on de Sitter and on black holes

Speaker: Georgi Dvali
Institution: Max-Planck-Institute, Munich
Time: Tuesday 09 February 2021, 13:00
Venue: Webinar
Abstract: We discuss how the S-matrix formulation of quantum gravity/string theory excludes any sort of stable or a metastable de Sitter. The constraint is enforced by the phenomenon of quantum break-time. This has a number of observable consequences both for early and late cosmology. In particular, it excludes cosmological constant as part of the energy budget in the Universe. We confront the quantum portraits of de Sitter and black holes and point out similarities as well as fundamental differences.
Recording: The recorded talk can be found here.

Consistency of supersymmetric 't Hooft anomalies

Speaker: Adam Bzowski
Institution: University of Crete
Time: Tuesday 16 February 2021, 13:00
Venue: Webinar
Abstract: We consider recent claims that supersymmetry is anomalous in the presence of a R-symmetry anomaly. We revisit arguments that such an anomaly in supersymmetry can be removed and write down an explicit counterterm that accomplishes it. Removal of the supersymmetry anomaly requires enlarging the corresponding current multiplet. As a consequence the Ward identities for other symmetries that are already anomalous acquire extra terms. This procedure can only be impeded when the choice of current multiplet is forced. We show how Wess-Zumino consistency conditions are modified when the anomaly is removed. Finally we check that the modified Wess-Zumino consistency conditions are satisfied, and supersymmetry unbroken, in an explicit one loop computation using Pauli-Villars regulators. To this end we comment on how to use Pauli-Villars to regulate correlators of components of (super)current multiplets in a manifestly supersymmetric way.

Einstein-Cartan gravity: Inflation, Dark Matter and Electroweak Symmetry Breaking

Speaker: Mikhail Shaposhnikov
Institution: EPFL, Lausanne
Time: Tuesday 23 February 2021, 13:00
Venue: Webinar
Abstract: It is well-known since the works of Utiyama and Kibble that the gravitational force can be obtained by gauging the Lorentz group, which puts gravity on the same footing as the Standard Model fields. The resulting theory - Einstein-Cartan gravity - happens to be very interesting. First, it incorporates Higgs inflation at energies below the onset of the strong-coupling of the theory. Second, it contains a four-fermion interaction that originates from torsion associated with spin degrees of freedom. This interaction leads to a novel universal mechanism for producing singlet fermions in the Early Universe. These fermions can play the role of dark matter particles. Finally, it may generate the electroweak symmetry breaking by a non-perturbative gravitational effect.
Recording: The recorded talk can be found here.

Two-Point Correlator of Chiral Primaries with a Wilson Line Defect in N=4 SYM

Speaker: Pedro Liendo
Institution: DESY, Hamburg
Time: Tuesday 02 March 2021, 13:00
Venue: Webinar
Abstract: We consider the two-point function of the stress-tensor multiplet in N=4 SYM and add a Maldacena-Wilson line to the configuration which makes the correlator non-trivial. We use a combination of perturbation theory and defect CFT techniques to obtain results up to next-to-leading order in the coupling constant. Being a defect CFT correlator, there exist two (super)conformal block expansions which capture defect and bulk data respectively. We present a closed-form formula for the defect CFT data, which allows to write an efficient Taylor series for the correlator in the limit when one of the operators is close to the line. The bulk channel is technically harder, nevertheless we use our analysis to check against well-known data of N=4 SYM. In particular, we recover the correct anomalous dimensions of a famous tower of twist-two operators (which includes the Konishi multiplet), and successfully compare the one-point function of the stress-tensor multiplet with results obtained using localization. Time permitting, we'll discuss work in progress on the strong coupling limit of the same correlator.

Tensor Models and Limit Cycles

Speaker: Christian Jepsen
Institution: Stony Brook, New York
Time: Tuesday 09 March 2021, 17:00
Venue: Webinar
Abstract: Matrix and tensor models with U(N) or O(N) symmetry offer a surprise on analytic continuation in N: the existence of fixed points of the beta functions where the the coupling constants gi are real but the scaling dimensions, as determined by the eigenvalues of the Jacobian matrix ∂βi/∂gj, are complex. When these complex eigenvalues cross the imaginary axis, a Hopf bifurcation takes place and gives rise to a limit cycle in the RG flow. In this talk, I will present an overview of this phenomenon and present a number of examples.

TBA

Speaker: Eric Perlmutter
Institution: Caltech
Time: Tuesday 16 March 2021, 13:00
Venue: Webinar
Abstract: TBA

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Speaker: Miguel Montero
Institution: Harvard
Time: Tuesday 23 March 2021, 13:00
Venue: Webinar
Abstract: TBA

Supergravity Amplitudes on AdS5×S5 and their flat-space limit

Speaker: Alessandro Georgoudis
Institution: IPhT Saclay
Time: Tuesday 30 March 2021, 13:00
Venue: Webinar
Abstract: We will study the four point function of the superconformal primary of the stress-tensor multiplet in four dimensional N=4 Super Yang Mills, at large 't Hooft coupling and in a large N expansion. This observable is holographically dual to four graviton amplitudes in type IIB supergravity on AdS5×S5. We first will construct the most trascendental piece of the correlator at any order in a large N expansion. They are determined only by leading order CFT data. By focusing on a specific limit, we make connection with the dual supergravity amplitude in flat space, where such structures correspond to iterated s-cuts. We make several checks and we conjecture that the same interpretation holds for supergravity amplitudes on AdS5×S5.

TBA

Speaker: Konstantinos Roumpedakis
Institution: Stony Brook
Time: Tuesday 06 April 2021, 18:00
Venue: Webinar
Abstract: TBA

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Speaker: Riccardo Rattazzi
Institution: EPFL, Lausanne
Time: Tuesday, 11 May 2021, 13:00
Venue: Webinar
Abstract: TBA